"off the top" in general and here means the first 5 hours
In "off the top of my head" it means the first idea that came into my mind.
Similar principle
This could easily be Norfolk dialect, though the word rummun has clearly been part of general British speech. It is nowadays a bit dated.
It means 'rum one', the adjective rum meaning strange, odd, different, peculiar etc.
A Norfolk expression Tha's a rummun ent it means 'It's a strange thing, isn't it'.
*They're rummuns' would mean simply 'they are odd/strange ones'. It can equally be applied to people as to objects etc.
Rum used as an adjective in this way is confirmed by the OED (sense 6) as dated, and they are uncertain of its etymology. It is predominantly British, but I wouldn't mind betting that many older-generation Australians, New Zealanders etc use it. A few of the more recent examples from the OED:
1955 J. Thomas No Banners vii. 61 ‘This is a rum go,’ Alfred said.
1977 J. I. M. Stewart Madonna of Astrolabe xi. 153 Some Scottish names are distinctly rum. Yours is.
1993 A. Habens in M. Bradbury & A. Motion New Writing 2 247 It's a rum do if a chap isn't allowed to remember what he remembers.
The adjective rum gives rise to may composites e.g. rum-looking, rum-sounding etc
Best Answer
Ariane is right and could have posted as an answer.
If you imagine hair, plaited into a thick braid, a pair of scissors isn't going to cut through it in one clean slice. Whoever is wielding the scissors will have to make a number of cuts before they make it through.
The writer is asking you to picture these scissors, opening and shutting like teeth, gradually but persistently cutting their way through the thick braided hair.