John Gill Commentary Job 41:23

"The flakes of his flesh are joined together: They are firm upon him; they cannot be moved." — Job 41:23 (ASV)

The flakes of his flesh are joined together
The muscles of his hefty are not flaccid and flabby, but solid and firmly compacted;

they are firm in themselves; they cannot be moved ;
that is, not very easily, not without a large sharp cutting knife, and that used with much strength.