ambush

Ambush comes from Middle English enbuschen, from Old French enbuscier, from en- plus Vulgar Latin boscus ("wood"), itself from Frankish *busk ("bush"), from Proto-Germanic *buskaz ("bush, heavy stick").

The word simply meant to hide oneself in a wood or thicket, and that literal act of concealment in brush became the word for a surprise attack. The shift from earlier en- to am- in English is unexplained.

It's a cousin of bouquet, which comes from the same Vulgar Latin boscus ("wood"); it's also related to ambuscade, a longer word for the same kind of hidden attack.