"Celtic monumental stone," 1768, from Welsh llech, cognate with Gaelic and Irish leac (see cromlech).
"yen, strong desire" (especially sexual and sometimes implying perversion), 1796, variant of letch, but according to OED "now regarded as a back-formation" from lecher. Meaning "a lecher" is by 1943. As a verb by 1911. Related: Leched; leching.