The sound a horse makes instead of a neigh, at least according to Spongebob Squarepants.
Found in the episode My Pretty Seahorse, where Spongebob finds Mystery the seahorse.
Mystery: neighhhhhhh
Mr. Krabs: What was that?
Spongebob: Just practicing my impression of Mystery, Mr. Krabs! Weesnaw!
Everyone seems to associate "Weesnaw" with that Spongebob episode with the pet seahorse Mystery. But there's also a scene from an episode of an old Winnie the Pooh cartoon series with the word "Weesnaw". In that scene, Pooh and his friends are in the grocery store, when Eeyore tests out a ballhorn that makes a loud "weesnaw" sound, and the grocery store clerk says:
Depends on the region of the United States. In the South and Middle America, it is an unmentionable body part. In the West it is a mythical sea horse. In the East, it is an exclamatory remark. It can also be used as a term of affection.
In the South: Uh oh, my weesnaw is really hurting. I better go get this checked out.
In the East: Weesnaw!! The sox won again!
In the West: someday, I hope to catch a Weesnaw.
Originated from the television show SpongeBob SquarePants, it evolved into garnering a sexual innuendo surrounding the term and eventually became a nickname for the genitals (usually male but can be attributed to female as well) or ejaculation.
"Dude, Victoria was sucking my weesnaw and I couldn't even get hard"
"Did you even cum?"
"Yeah, I weesnaw'd in her mouth soft!"