'boo' originates from imitative (echoic) English, specifically the interjection 'boo' used to startle or express contempt.

'boo' began as an interjection in English (mid-17th to 18th century) imitating a sudden shout to frighten or call attention; it later developed into the verb 'to boo' (to shout 'boo' at) and the noun 'a boo' (an expression of disapproval); the plural form 'boos' naturally followed for multiple occurrences.

Initially, it meant to startle or frighten by shouting 'boo'; over time it evolved to primarily mean expressing disapproval or contempt (and separately developed slang senses like 'boo' = beloved).