daisy

From Middle English dayesye, from Old English dæġes ēage, literally 'day's eye,' a name given because the flower closes its petals at night.

The name stuck around simply because the habit it described—opening by day, shutting by night—never stopped being true. Later, English speakers also used 'daisy roots' as rhyming slang for boots.

The word is built from 'day' and 'eye' fused together, so every time you say it, you're quietly saying 'day's eye.'