Old English læs (adv.) "less, lest;" læssa (adj.) "less, smaller, fewer" (Northumbrian leassa), from Proto-Germanic *laisizan (source also of Old Saxon, Old Frisian les "less;" Middle Dutch lise "soft, gentle," German leise "soft"), from PIE root *leis- (2) "small" (source also of Lithuanian liesas "thin") + comparative suffix.

From the first, the adverb has been used often with negatives (none the less). Much less "still more undesirable" is from 1630s. Formerly also "younger," as a translation of Latin minor, a sense now obsolete except in James the Less. Used as a comparative of little, but not related to it. The noun is Old English læsse.

less (adv.)

early 15c. as a shortening of unless. Extended contraction lessen, less'n, U.S. dialectal, is attested from 1881.

Others are reading

Definitions of less
less (adj.)
(comparative of `little' usually used with mass nouns) a quantifier meaning not as great in amount or degree;
less time to spend with the family
of less importance
less than three years old
a shower uses less water
less (adj.)
(usually preceded by `no') lower in quality;
no less than perfect
less (adj.)
(nonstandard in some uses but often idiomatic with measure phrases) fewer;
no less than 50 people attended
less than three weeks
in 25 words or less
less (adv.)
used to form the comparative of some adjectives and adverbs;
less expensive
less interesting
less quickly
Synonyms: to a lesser extent
less (adv.)
comparative of little;
she walks less than she should
he works less these days