noun
grump; complainer; whiner
Colloquial and slightly old-fashioned term for someone who constantly complains or grumbles. こぼす means 'to spill' or 'to complain', and 屋 is a suffix for a person characterized by that action.
うちの祖父はこぼし屋で、いつも何かに文句を言っている。
My grandfather is a grump; he's always complaining about something.
あのこぼし屋の話を聞いていると、こっちまで気が滅入る。
Listening to that whiner gets me down too.
The verb meaning 'to spill' or 'to complain'. こぼし屋 is derived from the complaining sense.
A more modern and common verb for 'to complain' or 'to grumble'. こぼし屋 is a noun for a person who does this habitually.
Compound of こぼし (continuative form of こぼす 'to complain') + 屋 (suffix for a person with a certain trait or occupation).