Meaning of “Nechikkoi(ねちっこい)”

“Nechikkoi(ねちっこい)” is an onomatopoeia.
The image of it is insistent or persistent, I think.

Image the gum adhered to the sole of the shoe, it’s difficult to remove it.
Additionally, the onomatopoeia to discribe the gum might be “NechoNecho(ねちょねちょ)” in my opinion.

Example Conversation

A: Why do you hate your boss?
B: I hate him because his personality is nechikkoi(ねちっこい).
He still blame me for the failure 3 years ago persistently

A: なんであの上司のこと嫌いなの？
B: 性格がねちっこいから嫌い。未だに３年前の失敗のことをしつこく責めてくるんだよ。