transitive (reflexive). To slim; to lose weight. Also (in later use esp.) intransitive in same sense. Now chiefly U.S.

intransitive. To make oneself slender, to slim.

intransitive. To try to reduce one's weight by dieting; to become slim. Also with down. Also figurative.

to lose weight: to become thinner or less corpulent; to put on weight: see to put on 6a at put, v. phrasal verbs 1.