Ruling 2330
The guarantor and the creditor must be of the age of legal responsibility (bāligh), sane (ʿāqil), and no one must have compelled them [to enter into the suretyship agreement]. Furthermore, they must not be foolish with finances (safīh),[21] and the creditor must not have been proclaimed bankrupt (mufallas). However, these conditions do not apply to a debtor; for example, if a person acts as guarantor for paying off the debt of a child, an insane person, or someone who is foolish with finances, it is valid.
-Ayatullah Sistani, Practical Laws of Islam, Suretyship (Ḍamān)
1
Ruling 2091 provides further clarification of this term: it refers to someone who spends his wealth in futile ways.