means To shine with a quick, flickering, intermittent light — or, of an eye, to gleam with amusement or delight.

from From Old English 'twinclian,' to wink or blink, a frequentative form built on a root meaning to flicker or vanish (a cousin of 'twitch' and the dialect 'twink,' a blink of an eye). The frequentative ending '-le' is the grammar of repetition itself — 'sparkle,' 'crinkle,' 'twinkle' — so the word's very shape stutters, doing the rapid on-off it names. The sense of a brightly amused eye is old too, the gleam treated as a kind of light winking on and off.