fill out
- 01 B2 Upper Intermediate
to become larger, stronger, or heavier as you grow up.
to develop a more substantial or rounded physique, typically through physical maturation or weight gain.
Simple“He was very thin as a child, but he started to fill out in his late teens.”
In context“After a summer of intensive training and a disciplined diet, the young athlete began to fill out, gaining the muscle mass needed for the senior league.”
UsageThis sense is intransitive and does not take a direct object.
contrast with 'fill in' (British English preference) and 'fill up' (used for containers like a glass or a gas tank); 'fill out' is the standard American term for documents.
In the sense of to complete a form, originally attested in American English; possibly as a calque of German ausfüllen. Later spread to British English, where it now competes with the traditional expression fill in.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.