On the website Ask Greek (which altan knows about), this answer is proposed:-
'λαϊκό means folk, common, ordinary, not sophisticated.
σκληρό means hard, stiff.
τσόφλι means shell (eg egg shell or nut shell).
It has to do with the character of the hero. What the author probably means here is “not sophisticated”.'
In Report to Greco Peter Bien translates it as ' I was coarse and taciturn, with the tough hide of a peasant'. Peter Bien's translation covers all the meanings given, with the extra advantage of fidelity to the original, like Pontios's suggestion.