Luckier means having better fortune. Luckier than most, a luckier throw, the luckier of the two.
The y turns to i before -er, as it does in happier and easier. People judge luck backwards, calling somebody lucky once the result is known, which is why luckier is nearly always said afterwards rather than in advance.
1adjectiveluckierHaving better fortune.
“My brother is always luckier than me; he even found money in the street.”
luckier is a comparison.
Middle English
lukky
English
lucky
English, today
luckier
lucky comes from Middle English lukky, equivalent to luck + -y. Cognate with Scots lucky (“lucky”), West Frisian lokkich (“lucky, fortunate”), Dutch gelukkig (“lucky, fortunate, happy”). Compare also Danish lykkelig (“happy”), Swedish lycklig (“happy, lucky”), German glücklich (“happy”), stq glukkelk (“happy”).
Etymology from Wiktionary, CC BY-SA