From a seaman's perspective, a lump is not that person (husband, child, brother-in-law) that spends most of their time on your couch, in front of your TV, eating. I know how you feel though.
The nautical usage of lump comes to us, as usual, from the navies in the great age of sail. A lump was a heavy, one decked boat that could carry large objects - anchors, etc. - out to ships from dock or levee. They also ferried ballast. In England, perhaps because of the similar shape of the boat and the fish, lump was the name for the baggety or owl fish known to naturalists as Cyclopterus lumpus. I love that name.
Lump was the term used for a sudden and unexpected fall from the rigging or slings which, doubtless, left one with at least a lump if not much worse.
Finally, our term lump sum for an entire payment seems to come from the dockside. A lumper in the late 18th and early 19th century was a worker who did a specific job or "lump work" rather like our "day laborers". For instance, they might load or unload a vessel. They were then paid a "lump". One payment for the job done rather than a weekly or monthly payment on an ongoing basis. The idea has morphed into the lump sum of money we think of today. Writers get paid a lump sum advance. I could use one of those about now.
So there you have it, Brethren. Call your bother-in-law a Cyclopterus lumpus during the Colts game tomorrow. Let me know what happens.