squint (adj.)
1570s, "looking different ways; looking obliquely," shortened form of asquint (adv.). Meaning "looking indirectly" is from 1610s.
squint (v.)
1590s, from squint (adj.). Related: Squinted; squinting.
squint (n.)
"non-coincidence of the optic axes," 1650s, from squint (adj.). Meaning "sidelong glance" is from 1660s.