noun
everyday clothes; ordinary wear
Refers to clothes worn in daily life, as opposed to formal wear or special attire. Often used in contrast to 外出着 (going-out clothes).
休日は内着で過ごすことが多い。
On holidays I often spend the day in everyday clothes.
内着と外出着を分けて収納している。
I store everyday clothes and going-out clothes separately.
noun
Used for garments worn under outer clothing, such as undershirts or slips. Less common than 下着 (したぎ) in modern Japanese, but still understood.
寒い日は内着を重ね着する。
On cold days I layer underwear.
この内着は肌触りが良い。
This underwear feels good against the skin.
Compound of 内 (uchi, 'inside') + 着 (ki, 'clothing'). The reading うちぎ is a regular rendaku form. The spelling 打ち着 is likely ateji or a variant.