My cousin has been to Australia and came back saying “fair dinkum” but he won't tell me what it means. If you will tell me I will surprise him next time I see him.
unquestionably good or genuine : excellent —often used as a general expression of approval
Nail him with this - typically, someone says it to conclude a set of simple instructions with "And Bob's yer uncle" - an example would be "okay, so yer digs yer hole, pop in the post, add some 'crete and bob's yer uncle.