「Dog at the insertion point of cavity」describes a male LUNGING FORWARD and PLUNGING IN given any opportunity.「Deep cavity」and「dog」or in Japanese fonts「cavity」with「two-handed, big」meaning「at home copulating in a cavity as does a dog」or「snared while copulating as is a dog」reflecting how aroused a male is with a wagging penis PROTRUDING and LUNGING as might a horny dog or a man with his spear, weapon, tool.
　Most likely though, a reference to a dog's BULGE while snared in the vaginal cavity during COITUS with THRUSTING, and the epitome throughout the animal kingdom of absolutely complete 'all in babe' vaginal PENETRATION: 'bitches' are female canids, and male canids, the 'dogs,' are endowed with a very special penis unlike any other species. A dog is equipped with two glands located at the base of the shaft that engorge and harden upon erection, normally after baculum penetration into the bitch's vaginal orifice (a baculum is a bone located within the dog's penis that enables rapid insertion, i.e., spearing). These locking bulbus glandis swell large enough to prevent the penis from being extracted from the bitch's vagina for quite some time, normally ten to fifteen minutes (or up to an hour in some cases), to allow the dog's semen sufficient time to get enough of a head start toward the bitch's ova that the male might actually sire some pups, rather than the next dog waiting to mount the same bitch in heat, with the penis remaining completely all in and snared from tip to below the base for the duration.
　With no prohibitions against bestiality in the old days, the literal elements of「at home copulating with domesticated dog」may apply literally, although it might be best if at all possible for the dog to have had a partner of a similar species. Urban legend has it that human females seem to have a penchant for big dogs more than human males do, with males exhibiting a general preference for ewe or pussy.
　Some insist the meaning as the lower element of「dog」LUNGING up and out from inside a roofed pit hole, PROTRUDING, STABBING, THRUSTING OUT its head. A dog might be almost unstoppable after being cooped up in a dark pit hole for some time. The dogs may certainly have been used as defense while the owner was away foraging, hunting for chicks and bunnies, or grocery shopping. Needless to point out is that the other listed meanings are not well covered by this explanation.
　Traditional fonts add an extra stroke that denotes the lower element as「dog」and not「two hands, big.」
　Some browsers will not display this glyph using certain fonts…the actual glyph may possibly be seen within other glyphs as an element.

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: to dash; to move forward quickly; to bulge; to protrude; to break through; to rush out; sudden
Japanese: thrust, stab, lunge, pass, tsuki, thrust to the throat, thrust to the chest, to prick, to stab, to poke, to prod, to push, to thrust, to nudge, to hit, to strike, to use, to prop oneself up with, to press against, to attack, to brave, to peck at, to pick at, to egg on, to put up to, to pierce, to push out, to project, to stick out, to hand over, to cross, to go across, to go through, to cut across, penetration, digging into something, straight man, retort, quip, sex, intercourse
Unihan extended: suddenly, abruptly, unexpectedly
EDRDG: stab; protruding; thrust; pierce; prick; collision; sudden