1. To be worried or scared about something, frequently used to describe a drug-induced state of paranoia.
2. To vomit.
Origin: Great Grimsby. Possibly originally evolved from the word 'defecate', via 'shitting it'.
1. 'I'm deffing', 'She's deffin'-out', 'I deffed-out', 'Deff-out'
2. 'I just deffed'. 'He's deffin''.