Lugging is more of a term of art than science ... so defining it as folks have done above helps.
Traditionally, Europeans think lugging is letting the RPMs go below 3,000. A friend of a friend ate up his Porsche's flat six driving like that ... because his mechanic told him to do it.
I tend to drive at a very low RPM ... just above where the engine would struggle. My minimum speeds in gears for my stock '06 Civic are as follows (from memory):
5th 36mph
4th 24mph
3rd 13mph
2nd 7mph
I'll have to see where the RPMs are in these speeds 'n gears.