What does "trustless" mean in crypto systems like Bitcoin and Ethereum?

What does "trustless" mean in crypto systems like Bitcoin and Ethereum? The ”trustless” part is primarily the public-private key signatures, replacing human oversight to authenticate transactions, no need to trust a human to know that a transaction took place, ”key-to-key”.

This here is the earliest scientific work on mathematical signatures, 1976, from Diffie Whitfield and Martin E. Hellman,

"have given rise to a need for new types of cryptographic systems, which [...] supply the equivalent of a written signature. "