Hello Nurse is a running gag in Animaniacs and one of Yakko and Wakko's catchphrases. Whenever Hello Nurse or another attractive girl walks in the room, Yakko and Wakko both go "Hellooooooo Nurse!" and go nuts over the girl. Dot just criticizes them for doing that, but whenever an attractive guy comes in, she does the same thing. This is a running gag that has happened throughout the entire series.
This phrase originated in vaudeville. It was originally going to be Buster Bunny's catchphrase along the lines of Bugs Bunny's "Eh, what's up, doc?" but the writers could not find a way for him to use the phrase.
De-zanitized: Yakko and Wakko do the Hello Nurse thing for the first time in the series when Dr. Scratchandsniff brings the nurse into the room. Hearts are in their eyes and they imagine going on dates with her. Dot imagines doing it with Dr. Scratchandsniff and kissed him at the beginning of the episode.
Cookies for Einstein: When Wakko sees a hot Swiss girl go by he says "Helloooo Swiss Nurse Miss!"
Taming of the Screwy: Yakko and Wakko do the hello nurse thing when Dr. Scratchandsniff brings out The Nurse so they can practice greeting people for the party.
When The Warners are practicing to eat dinner and the Nurse walks in as their servant, they do the hello nurse thing. Dot begins to criticize them, but then stops herself because she doesn't even want to bother doing so.
At the party, Dot says "Helloooo Mr. Lethal Weapon" and then kisses him. Yakko and Wakko appear in a bowl of milk in front of Catwoman and say "Helloooo Kitty!"
The Girl With The Googily Goop: When meeting Googi for the first time, Yakko and Wakko say "Hellooooo Googi!"
Temporary Insanity: Yakko says "Hello Nurse!" when he gets the phone.
When Mel comes in while Dot is putting everyone on hold Dot says "Hellooooo Mel!"
Yakko said Hello Nurse again when he picked up the phone.
Piano Rag: Dr. Scratchansniff trips up his instructions to Hello Nurse with a fantasy.
When Hello Nurse stands over Yakko with a net, Yakko slams the door while saying "Goodbyeeeeeeeeee Nurse!" This was also used as the closing tag for episode 7.
Bumbie's Mom: Hello Nurse cameos, with Yakko, Wakko, and Dot cameoing immediately after Slappy asks the Nurse for a sedative and the two brothers saying, "Helloooooo Nurse!"
King Yakko: Yakko suggests to Prime Minister Nurse repeatedly over the course of the cartoon that he would like to get personal with her, though only saying "Hellooooo Prime Minister Nurse!" in the beginning.
Garage Sale of the Century: A couple of girls walk past the Warners while they are standing still in front of the garage with an angry and confrontational Pa Bear, which the brothers respond to with their signature "Helloooo Nurse" and Dot responds to with a testy look. Wakko tries Pa Bear's remote control which he's discovered can manipulate the movement of things he points it at, but Dot stops him from doing so.
La La Law: Yakko's introduction scene involves him chasing an office worker around a table, with those hearts in his eyes that are seen in De-Zanitized.
Space Probed: Yakko thinks of Hello Nurse on the spaceship, which causes her to appear and lead him to the control room (against her will of course)
Meatballs Or Consequences: When Yakko ask the Grin Reaper if they could watch the adult channel, he and Wakko said "Helloooo Nurse"
Hearts of Twilight: They manage to sneak in Yakko "Helloooo Large Nurse!" on appearance of the fat woman in the shoe shop.
Moon Over Minerva: Minerva gets the same reaction from most of the male animals as Nurse does from the Warner brothers. To contrast, Wilford's werewolf form gets the same reaction from most of the female animals (including Minerva) like Mel Gibson gets from Dot.
Baghdad Cafe: Slappy (obviously very confused by this episode) gets the Iraqi guard to boost her onto his arms and says "Helloooooo... Nurse." She then looks back on the script with a quizzical expression on her face, and remarks that she doesn't get it.
Survey Ladies: A couple of girls walk past them. "Hellooooo, 90210s!" say Yakko and Wakko.
Windsor Hastle: When the Warners first see the Queen of England, they shout "HELLOOO, QUEEN!" They also kiss her on the cheek, making it the same ritual as with Hello Nurse herself.
A Christmas Plotz: Yakko, in role as the ghost of Christmas Yet-to-Come, says "Hello, nurses!" when they converge on Plotz and Yakko.
The Warners and the Beanstalk: The Warners spot the harp, and do that strange Hello Nurse thing with hearts in their eyes, at which point Yakko remarks that she's a dream, Wakko says she's a princess, and Dot points out that she's a woman with a harp stuck to her back.
Deduces Wild: When the warners show Sherlock homes and Dr. Waston the picture of the cast of Baywatch, they all said "Hello Nurses on the beach!"
A Hard Day's Warners: After the Warners left from the fan they came back through the curtain and said "Hello Nurse!" and the fans started chasing them.
The Kid in the Lid: according to the story, Wakko and Dot kiss the boy and girl and said "Helloooo, Nurse!"
Star Truck: Mr. Spock saw Uhura come through the door and says "Hellooo Nurse!"
Hello Nurse (song): This mostly used the gag during the song what rhymes with nurse.
Cutie and the Beast: During the dot song, Yakko and Wakko pop out of a pocket of long underwear and said to the woman "Hello Nurse"
A Very Very Very Very Special Show: A bunch of men on the beach we're attracted to Hello Nurse.
Acquaintances: When Yakko asks "Where in the world is Carmen Sandiego gonna live?", Carmen Sandiego passed by and wake says "Hellooo Nurse!"
Star Warners: Wac Skylinker said "Helloo space case nurse" to a Unjawa when she and 3-pinkEO and Brain2-me2 arrived at his farm.