Wood half burnt is easily kindled.

Something already partly consumed, weakened, or set in motion may be more readily driven to its next stage. The image suggests that a prior condition can make renewed action easier than beginning from an entirely untouched state.

Applied to people or situations, the saying may neutrally describe an unfinished process that is easily revived. It can also caution that earlier conflict, emotion, or damage leaves matters ready to flare up again when a fresh cause or incentive appears.

Use it literally of fuel that has already caught fire, or figuratively when prior agitation or partial change makes renewal or escalation easier. Its fire image suits reflective discussion of disputes, passions, or processes; context determines whether the emphasis is readiness or warning.