Adjective. A particularly intense or extreme experience, whether good or bad.
"The party got a bit full-on when those randoms turned up with all that crystal meth."
"He told me he loved me...I've only known him three weeks so it was all a bit full-on."
<adverb> British Slang;
1) Synonymous with 'absolute', 'positive', 'total' etc...
"Jonathan is an absolute, full-on knob-head!"