Heft is the bulk or weight of a person or thing. You might shift the heft of that huge bag of dog food you just bought over to your left arm while you use your right hand to open the door.
The heft of your big suitcase might make you question the wisdom of packing so many books. You can use heft as a verb, too — you could move the books into your backpack and then heft it back onto your shoulder. Heft comes from the verb heave, "lift with effort," modeled on verb/noun combinations like "thieve" and "theft" or "weave" and "weft."