The term is most commonly referred to as,
1: a physical act performed by the male partner, immediately preceding sexual intercourse in a rear-entry position.
2: To position one's self to sexually penetrate his partner, specifically in a back-entrance position, otherwise known as 'doggy-style' (common slang).
3: To prepare for copulation.
4: When a stallion (a male horse) positions itself on top of the female horse, just prior to breeding. This pre-penetration act is known as nucking.
In a 'doggy-style' position, the male is typically standing on his two legs and supporting himself with one of his arms either on the floor or the female's back. Meanwhile, he is stabilizing himself by adjusting his footing/balance, if you can picture that. The male is nucking, getting ready to penetrate the female. It is the most natural, primitive state of sexual intercourse between male and female. (i.e., as the beast nucked in preparation for copulation, the excruciating anticipation of the pleasure awaiting him caused his erection to jump and ooze with pre-ejaculate fluid.)