«ثُمَّ اسْتَنَّ اسْتِنَانَ الْجَوَادِ فِي الْمِضْمَارِ»
He then dashed off like a fine horse racing on a track.
Istanna evokes vigorous rapid movement. Al-jawād here means a noble or swift horse, and al-miḍmār a racing or training ground.
The pairing istanna istināna uses a verb and its cognate verbal noun for emphasis. There is humour in the scale of the comparison: an ordinary trip for a bath is described like a race.