When we say "batting an eye", "opening an eye", etc it's clear that we're talking about eye lids since the alternative would be very painful and make no sense.

Colloquially, when we refer to "eyes", we're almost always referring to the facial structure through which the eyeball is visible and almost never the literal eyeball itself.

I mean, when you say somebody has attractive eyes, surely it's understood that you're not referring to the spherical ball of goo itself, but rather their facial structure.

Once you have made your guess as to the meaning, then and only then link to Wikipedia for a marvelous explanation from a not-so merry old England.