When someone doesn’t believe you so they say “say mums” to make sure you’re not lying

A request for the announcer of an unverified fact or story to "swear on their mum's life".

A statement used to replace a term such as "Swear to God" to really put the point across that you're telling the God honest truth. If it's obviously a lie and someone claims "mums", their mum is thought to die.

It is another form of saying swear on my mums life, by saying say mums you are trying to prove something