noun-modifying (irregular), na-adjective
hack; incompetent; useless; quack
Colloquial and mildly derogatory. Describes a person or their work as unskilled, amateurish, or of poor quality. Often used attributively (へっぽこな〜) or as a noun modifier (へっぽこ〜). The 'quack doctor' sense is a specific application.
あのへっぽこな整備士に車を直してもらうのは不安だ。
I'm uneasy about having that incompetent mechanic fix my car.
彼
He was seen by a quack doctor and his condition worsened.
The origin is uncertain. It may be a sound-symbolic word (擬態語) with the へっ- prefix suggesting something weak or inferior, similar to へなへな or へたれ, but the exact derivation is unclear.