Källarlucka is ”trapdoor to a cellar”. Källare is ‘cellar’ or ‘basement floor’ . Lucka is either an opening, gap, hole, or that which covers said opening: a small door, a hatch, or in this case a trapdoor.
Lucka is derived from the Old Swedish verb luka, Germanic *lukan ‘to shut’. From this comes also the English verb lock. Also Swedish lycka ‘small field’ and nyckel ‘key’ A closely related (by Ablaut) Germanic noun has developed into Swedish lock ‘lid’, English noun lock and German Loch ‘hole’.
Other relatives could be Swedish lycka ‘loop of thread'), and lycka, English luck (a twist of Fortune), and a lock of hair. The root to all these is probably Germanic *luk-, Indoeuropean *lug- ‘bend’, ‘twist’.