Where does “rigidissimo” come from?

rigidissimo (Italian) comes from Italian rigido, from Latin rigidus, from Latin -idus, from Proto-Italic *-iðos.

Ancestry of “rigidissimo”, step by step

Step	Language	Word	Meaning
1	Italian	rigido	rigid, stiff, inflexible, unbending, hard; rigid,...
2	Latin	rigidus	stiff, rigid, hard, inflexible; stern, strict,...
3	Latin	-idus	tending to
4	Proto-Italic	*-iðos	—