'bluish-green' originates from Modern English as a compound of 'bluish' and 'green'. 'Bluish' itself comes from 'blue' + the suffix '-ish' (meaning 'somewhat' or 'having the quality of'), and 'green' is the common English color word.

'bluish' developed from the adjective 'blue' (Middle English bleu/bleu(e)) plus the adjectival suffix '-ish' in later Middle/Modern English. 'Blue' goes back to Old English 'blāw' (via Proto-Germanic *blēwaz). 'Green' comes from Old English 'grēne' (from Proto-Germanic *grōniz). The compound 'bluish-green' arose in Modern English to name an intermediate hue between blue and green.

Initially the elements simply meant 'somewhat blue' and 'green' respectively; combined as 'bluish-green' it originally denoted any color intermediate between blue and green and has retained that basic meaning in modern usage.