not a spoiler. in korea, “thief” is just a slang/term used to someone who got smtg going on with a much younger person than them (like obvious big age gap). in this case, he didn’t called uke as thief, he actually said that to seme instead bcs he’s much older than uke (can refer to early chapters and in between their sex scenes where seme kept mentioning their obvious age gap) ^^
more than 10 years! it’s not mentioned in manhwa yet and seeing as it’s already ended (for S1) in korea, the author already answered your question on their Q&A section notes so you can refer the sources here, i translated it roughly on description ^^
https://imgur.com/a/MHxUdUi

why is the four eyes calling uke a thief?? any spoiler?