Expressions (phrases, clauses, etc.), Godan verb with 'su' ending
to give a warning; to remind (a person) of; to drive in a nail
Idiomatic expression
実際はそんなことあるわけないが、釘を刺すにはちょうどよさそうだ。There's no way something like that would really happen, but it seemed just right to drive the point home.
Also written 釘をさす【くぎをさす】、クギを刺す【くぎをさす】