  In a group $G$, a weak Sierpi\'nski subset is a subset $E$ such that for some
$g,h\in G$ and $a\neq b\in E$, we have $gE=E\smallsetminus \{a\}$ and
$hE=E\smallsetminus \{b\}$. In this setting, we study the subgroup generated by
$g$ and $h$, and show that it has a special presentation, namely of the form
$G_k=\langle g,h\mid (h^{-1}g)^k\rangle$ unless it is free over $(g,h)$. In
addition, in such groups $G_k$, we characterize all weak Sierpi\'nski subsets.
