1
general::
Third-class mail, such as unsolicited advertisements and flyers, that is sent indiscriminately. For example, While we were on vacation the front hall filled up with junk mail. [c. 1950]
American Heritage Idioms
2
general::
Phrase(s): junk mail
annoying, unsolicited mail, such as promotional letters, etc. • I am so incredibly tired of getting pound after pound of junk mail every day. I could just scream.
McGrawhill's American Idioms And Phrasal Verbs