ambushyoudaoicibaDictYouDict[ambush 词源字典]
ambush: [14] Originally, ambush meant literally ‘put in a bush’ – or more precisely ‘hide in a wood, from where one can make a surprise attack’. The hypothetical Vulgar Latin verb *imboscāre was formed from the prefix in- and the noun *boscus ‘bush, thicket’ (a word of Germanic origin, related to English bush). In Old French this became embuschier, and when English acquired it its prefix gradually became transformed into am-.

In the 16th century, various related forms were borrowed into English – Spanish produced ambuscado, Italian was responsible for imboscata, and French embuscade was anglicized was ambuscade – but none now survives other than as an archaism.
=> bush[ambush etymology, ambush origin, 英语词源]
bivouacyoudaoicibaDictYouDict
bivouac: [18] Bivouac appears to be of Swiss- German origin. The early 19th-century writer Stalder noted that the term beiwacht (bei ‘additional’ + wacht ‘guard’ – a relative of English watch and wake) was used in Aargau and Zürich for a sort of band of vigilantes who assisted the regular town guard. Beiwacht was borrowed into French as bivac, and came to English in a later form bivouac.

Its original application in English was to an army remaining on the alert during the night, to guard against surprise attack; in so doing, of course, the soldiers did not go to sleep in their tents, and from this the term bivouac spread to ‘improvised, temporary camp’, without the luxury of regular tents.
=> wake, watch
supprise (n.)youdaoicibaDictYouDict
mid-15c., "injury, wrong, outrage," from supprise (v.) "overpower, subdue, put down; grieve, afflict" (c. 1400), also "take unawares, attack unexpectedly" (mid-15c.), from Anglo-French supprise, fem. past participle of supprendre, variant of sorprendre (see surprise (n.)). The noun later also had sense "oppression; surprise attack," but perhaps originally was an alternate form of surprise used in a specific sense.