the.com/understatement

means A statement that deliberately or unintentionally expresses something as less significant, intense, or serious than it actually is.

from Straightforward carpentry of English parts: 'under' (below, less than) plus 'statement,' from Latin 'status' via Old French 'establir' — the act of setting something forth. The word simply does what it describes: it states, but under. First recorded use dates to the 1700s, when English was busy bolting 'under-' onto all sorts of words (undercook, underpay, undervalue) to mean 'insufficiently.' No hidden drama in the roots — the cleverness lives entirely in how the word gets used, not where it came from.