A derogatory colloquial term meaning unskilled or clumsy; a rude way to say someone is bad at something.
ギターがへたくそで、みんなに笑われた。
I'm lousy at guitar and everyone laughed at me.
字がへたくそだから、読みづらいよ。もっと丁寧に書いて。
Your handwriting is awful, so it's hard to read. Write more neatly.
へたくそと言われても、練習すれば上手になるよ。
Even if someone calls you lousy, you can get better with practice.