すっぽ ける
supponukeru

Ichidan Verb

1. to slip out (e.g. from one's fingers)

「刀 かたな投擲 とうてき練習 れんしゅう?」「すっぽ けただけです」
"katanano tōtekino renshū" "supponuketadakedesu"
"Practising sword throwing?" "It just slipped out of my hands."
Ichidan Verb

2. to cleanly forget