  Let $G$ be a finite group, let $p$ be a prime and let $w$ be a group-word. We
say that $G$ satisfies $P(w,p)$ if the prime $p$ divides the order of $xy$ for
every $w$-value $x$ in $G$ of $p'$-order and for every non-trivial $w$-value
$y$ in $G$ of order divisible by $p$. If $k \geq 2$, we prove that the $k$th
term of the lower central series of $G$ is $p$-nilpotent if and only if $G$
satisfies $P(\gamma_k,p)$. In addition, if $G$ is soluble, we show that the
$k$th term of the derived series of $G$ is $p$-nilpotent if and only if $G$
satisfies $P(\delta_k,p)$.
