Enter by force, as in The thieves broke in through the back door. [Mid-1500s]
Also see break into.
Also, break in on. Interrupt or disturb something unexpectedly, as in His assistant broke in with the bad news just as we were ready to sign the agreement, or He broke in on our private talks. [Mid-1600s]
Train or instruct someone in a new job or enterprise, as in Every semester she had to break in a new teaching assistant. [Late 1700s]
Loosen or soften with use, as in It takes a while to break in a pair of new shoes.