"knowing, wide-awake, untrusting, suspicious, alert," 1718, originally slang, with -y (2), but otherwise of unknown origin. Perhaps from dialectal lere "learning, knowledge" (see lore), or from leer (v.) in a now-obscure sense "walk stealthily with averted looks, sneak away" (1580s). OED suggests connection with archaic leer (adj.) "empty, useless," a general Germanic word (cognate with German leer, Dutch laar), of unknown origin.
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