the.com/boo

means An informal term of affection for a romantic partner, and also the sound you make to startle someone — two unrelated meanings sharing one short word.

from The affectionate 'boo' likely comes from French 'beau' (meaning sweetheart or boyfriend, literally 'handsome'), filtered through African American Vernacular English into mainstream slang. The startling 'boo,' meanwhile, is much older and probably imitative — a sharp burst of sound chosen simply because it's quick and jarring to say. There's a persistent folk claim that 'boo' descends from the Greek word 'boe' (a loud cry), but this is unverified and most etymologists treat the exclamation as independently formed from the sound itself, not inherited from any single ancestor word.