The word is a straightforward compound formed in 20th-century American English. Its logic is purely descriptive, deriving from the garment's resemblance to the capital letter 'T' when laid flat, due to its simple construction with a straight body and perpendicular sleeves. The second element, 'shirt', originates from the Old English 'scyrte', meaning a short garment, related to the Old Norse 'skyrta' (a shirt) and ultimately from the Proto-Germanic 'skurtijǭ'. This compound name effectively replaced earlier terms like 'tee shirt' or 'under-shirt' to describe this now ubiquitous item of casual wear.

Structure: T (shape) + shirt (garment) T (English, letter representing shape); shirt (Old English, base noun for a garment for the upper body)