ㄴ가 쓰다

write or use something-or-other

This is not a fixed idiom but a vague word running into the verb 쓰다. The vague words are 뭔가 and 무언가 (something), 누군가 (someone), 어딘가 (somewhere). 쓰다 covers both writing and using, and both appear here: someone writing something during a break, taking on an item somebody else used, feeling you have no skill worth using. 쓸만하다 means 'decent, worth using'.

Register: everyday