This shirt is made of flax fiber.
flaxseed; linseed
linen; linen cloth
flax yarn; linen thread
flax color; beige; ecru
flaxen; linen; made of flax
Standard kanji spelling for flax/linseed.
Katakana spelling, common in botanical or commercial contexts.
The kanji 亜麻 are ateji (phonetic equivalents) for the plant name; the reading あま is of uncertain origin, possibly from a foreign source.