it has two meanings tho one meant by the tweet its monday meaning back to work, the daily grind of hard work. the other is a form of dancing? sexualy (sugestive):in Gorge Carlin voice mind you god rest his soul" where people grind against each other in a club environment or wherever people have had too much to drink, and look like they are trying to get it on with their clothing still on, it is never classy, should never be done by any one but a striper male or female. but the tweet meant working hard back to work. the perv comment was thinking you meant the very explicit sexualy activity of drunk people at clubs

"Time to grind" is not a standard expression in the UK, but the original tweeter probably meant "work". "Work" can be called "the daily grind". If you get back to work after the weekend or a break, you can say something like : "Back to the grindstone" (the stone used to grind corn or wheat into flour).