'Practicing throwing swords?' 'It just slipped out of my hand.'
Literal
'Sword [possessive-の] throwing [possessive-の] practice?' 'Slipped-out only [copula-です]'
投擲 'throwing/hurling' — a formal military/martial term used here. すっぽ抜ける 'to slip right out (of one's grip)' — an expressive verb with the emphatic すっぽ prefix suggesting a clean, unintended release. The exchange is humorous — the second speaker's excuse doesn't quite convince.