Derogatory name for Russian, used in Ukrainian, Polish and Belarusian but also sometimes in Russian (usually to add a quaint Ukrainian note). Historically the term was used for Muscovites, and/or soldiers, without negative undertones but acquired that meaning as the Russian empire expanded westwards in the 18th century. The Ukrainian verb москалить means “to cheat” (in trade).
Ведь ещё в семнадцатом веке было на территории Украины дикое поле, по которому вольно гуляли казак с крымским татарином, отряд ясновельможных поляков в поисках приключений или осторожный москаль пробирался с отрядом, строя коварные планы.
In the seventeenth century there was a wild field in Ukraine where Cossacks and Crimean Tatar roamed free, along with the odd detachment of Polish noblemen in search of adventure or a cautious Moskal sneaking around with his crew plotting treacherous plans.
(Эдуард Лимонов, У нас была Великая Эпоха, 1987)