A phrase that has obtained popular use in areas of the UK containing northerners, chavs, or rural folk. It means "something or whatever", usually used in an inquisitive sense or as a dismissal of lack of knowledge following an observation. Irritating to hear for anyone who speaks real English.
D'you wanna come dahn 'ere 'n watch summint or summat?
He was blatherin' on 'bout jobs or summint or summat, don't really give a fook.
It shot roight over t'hill, loike an alien or summint or summat! Roight fooken frit me.