I’d say you were right about modern colloquial usage, Father Steve – it’s not an obvious choice. But OED2 traces its history as apparently quite an old form, quoting Hexham from 1648 and Emerson from 1856 ('Older than all epics and histories, which clothe a nation, this undershirt sits close to the body.' It also draws attention to the NFris onnersjürt, Da. underskjorte, and Sw. -skjorta.) for comparison.