burn
烙 centers on applying heat to mark or sear something: burning, branding, or cauterizing.
烙 combines 火 (fire) with 各, which likely contributes the sound. The character expresses burning or branding with fire.
The left side is fire 火, and the right side 各 looks like a grid or a brand. Picture a hot iron brand pressing a grid-like mark onto something, burning it in.
For ラク, imagine a red-hot brand sizzling as it burns a mark: the sound 'rack' like a branding rack cues ラク.