British slang for penis; also used informally as part of 'willy-nilly' meaning haphazardly or without planning.
The standalone word likely comes from the given name William used as a diminutive or familiar term. In the phrase 'willy-nilly,' it comes from 'will he, nill he' (will he or won't he), meaning whether willing or not.
The phrase 'willy-nilly' is actually frozen Old English grammar—'nill' is the opposite of 'will,' so it literally means 'will-not-will,' the ancestors of English speakers loved combining opposites to mean 'either way you look at it.'
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