A "meanie" is a mean person, someone who is unkind to others For example, a bully is a meanie. A bully would call another person rude and unkind names, or possibly hurt someone physically.

a villain or criminal in a story, movie, etc. baddie (or baddy), beast, brute, caitiff, devil, evildoer, fiend, heavy, hound, knave, miscreant, monster, nazi, no-good, rapscallion, rascal, reprobate, rogue, savage, scalawag (or scallywag), scamp, scapegrace, scoundrel, varlet, villain, wretch. Source: https://www.google.com/search?hp&ei=HirMW_7DNJCQ_QbuuZqIDA&q=meanie+synonym&oq=meanie&gs_l=psy-ab.1.5.0l9j0i10.3613.3613..8788...0.0..0.266.266.2-1......0....1j2..gws-wiz.lArSB7_Lm2Y

The expression "meanie" is generally NOT used for serious actions. It is often used when people don't go along with what you want to do. For example, your friend says that they have to leave the party and if you want a ride home you have to come NOW. You might say, "don't be such a meanie, can't we stay another 10 minutes".

“Sinister” is a little too strong. A “meanie” is just as the others described—a mean person. “Sinister” would be more along the lines of an evil person.

Meanie is an expression used by children, it would be odd for an adult to call someone bad a meanie.