He had a bug under his saddle

The speaker was describing a friend who was complaining. This is a congruent conflation of “burr under (one’s) saddle” and “bug up (one’s) ass”, both referring to a source or cause of a persistent annoyance. This one is similar to a previous post, “he put a burr in her bonnet”. https://malaphors.com/2012/12/28/he-put-a-burr-in-her-bonnet/ A big thanks to Martin Pietrucha for sharing this one.