Diceware is supposed to be done with Dice in an entirely offline manner. I guess a local program is still useful, but in theory the program could be a shoddy RNG with non-random passwords. The program shouldn't be trusted unless you wrote it yourself, or otherwise performed a security audit yourself.

For all of its faults, 5x six-sided dice looking up this wordlist (https://theworld.com/~reinhold/diceware.wordlist.asc) is something that can be accomplished by hand, without computer assistance.

Assuming you have trustworthy dice (they don't have to be perfectly balanced, just something you can trust to be random), then you can diceware your own password by hand.