Truncated form of "simma down now"; used specifically when trying to hold one's keep, defending against someone. Synonymous with "hold!!" or "wait!!"
in Lord of the Rings movie part II, when everyone's preparing to hold the fortress, you can at one point hear a determined "hooold!!, hoooolddd!!! hooooooollllld!!!!!". Well the alternative to that is
"simma!!!, simmmmmmaa!!!!!, simmmmmmaaaaaaa!!!!!!"