My partner have this hard, immovable, painless lump on his lower back. It hasn't gotten bigger as far as I know (for around 0,5-1 year). But it do concern me, as he won't see his GP.
It's deep in the tissue between the ribs and pelvis on the left side. It can only be palpated, not seen. It's a bit elongated, but only maybe around 0,5x1 cm. May be bigger, it's hard to know. It's not very well distinguised from the other tissues.