'Dekko'/'Decko' comes from the Hindi 'dekho' meaning 'look!'
Late 19thC and clearly brought back from India by British troops.
The phrase 'Have a decko' later became even shorter….
'Have a deek'
You aint been 'avin' a deek at nuff (British made) b/w war films – Standard Sunday afternoon TV matinee fodder for years on the Beeb.
Them squaddies were always having a decko under the bonnet when their engines packed in.