The act of agreeing to somehting
"Josh that chick sure has nice tits!"
It's a more agreeing version of the word Indeed, if you feel very strongly about agreeing about something that concerns your life then Indedery is usually the word you use for it.
Timmy - "What would you rather have? Cancer or Aids?"
Bob - "I kinda want AIDS, because then I'll have bunch of needles that are filled with my blood in a public masturbatory trench coat and stab people with them"
Milton - "Indedery"