Tardy is fine. It’s a different word. As for retard, I think it depends to whom you are referring to. Wouldn’t refer to a physical or mentally handicapped person as a retard. It was the same when I was at school. We’d call each other spastics or flids. But I guess, etymologically these terms are not “politically correct” but that’s language for you and meanings can change depending on the context and company in which they’re used. Bastard is another word that has “evolved”.
Discussion
Same etymology. Root word is tard. Latin for Slow
The word is used all over the place.
Tardigrade, tardus, [fire] retardant, tardy, etc
Definitely a shared etymology and used within other words, as with so many words in the English language that have developed from previous words, especially those derived from ancient German (Old English) and French/Latin (Modern English). To me, tardy is a distinct word from retard/retarded when it comes to modern day usage/definition/meaning - slow (in action) rather than referring to a mental or physical disability. That all said, I think “retard” is evolving a new nuanced meaning here on #nostr 🤣