understate

verb (used with object), un·der·stat·ed, un·der·stat·ing.

1. to state or represent less strongly or strikingly than the facts would bear out; set forth in restrained, moderate, or weak terms: The casualty lists understate the extent of the disaster.

verb

1. to state (something) in restrained terms, often to obtain an ironic effect
2. to state that (something, such as a number) is less than it is

v.1824, from under + state (v.). Related: Understated (of fashions, etc., from 1957); understating.