Mastrid is a jargon-hipster borrowing of the English phrase Must Read , which translates as "compulsory reading": Must - must, Read - read. In Russian it looks like a declined masculine noun. A stress on the letter "and".
Especially this word is fond of Alexei Navalny, regularly publishes fascinating mastridas on his site, that is, articles that every self-respecting oppositionist simply owes to read. Taking this opportunity, we send greetings to Navalny.
Mastrid is in the list: Slang