Wiktionary
vb. eye dialect misspelling and mispronunciation of must have or must've.
Usage examples of "must of".
But he who desireth to walk at liberty with Me, must of necessity mortify all his evil and inordinate affections, and must cling to no creature with selfish love.
Jeez, must of been a dozen guys came and went while the game was on.
Indeed, this mental repast is a dainty, of which those who are excluded from polite assemblies must be contented to remain as ignorant as they must of the several dainties of French cookery, which are served only at the tables of the great.
His first opinion was, that the savages in the boat never could live out the storm which blew that night they went off, but must of necessity be drowned or driven south to those other shores, where they were as sure to be devoured as they were to be drowned if they were cast away.
Looking down at them, he thought of what pain they must of endured.
The cold was the chill of the grave, and it carried the dusty must of an old tomb newly opened.
Yes, I used the word 'opportunities,' and as military planners, we have to be just as aware of those as we must of threats.