I think I can help you out with that, the slang term for 'seriously' is probably 'fer shure', but it's a bit old school, a more common one these days is 'for real', however, in crosswords, it's often shortened to 'fer real' or just 'fr', but another possibility is 'no joke', it really depends on the length of the answer, if you give me the number of letters I can try to narrow it down for you.