an informally performed surgery, usually without benefit of anasthesia, sterilization, or any kind of hygienic preparation. similar procedures include field castration, field lobotomy, field amputation, etc. in all cases the patient is someone deserving of rough treatment, and the surgeon is the person providing it.
harry: did you hear about cary? his girlfriend found out he was cheating on her, so she gave him a field vasectomy.
larry: ouch!! did she use any instruments?
harry: well, mostly she used her hands, feet, and knees. she did threaten to perform a field castration with a rusty spoon if she caught him cheating again.
larry: jeez, what a dumb fuck. he should have worn a cup.