I learned the term only a few days ago but I didn’t google it. It seemed to be a very old-fashioned phrase and very English. I googled it right now. A Redditor said

Calling someone an “old sock” is a thing, but not meant as a insult. It’s used for old people in an endearing way.

Don’t ask me how - but I had ended up on the Wikipedia page for “Old Sock”. It made me laugh and it seems like something that isn’t real but a comedy skit.

Old Sock (2013) is the nineteenth solo studio album by Eric Clapton. […] He was inspired to name the album Old Sock from a conversation with David Bowie. After hearing Bowie’s then-new song “Where Are We Now?”, Clapton sent a note thanking him “for writing such a beautiful song”. Bowie responded, “Thanks for the shoutout, old sock. Really appreciate it.” Bowie allowed Clapton to use the phrase for his album title and share the story of their conversation. Following Bowie’s death in 2016, Clapton wrote “Sweet dreams old sock” on his Facebook account.

The album cover is a selfie Clapton took with his iPhone while on holiday in Antigua.