Mendicus was originally just a general term to refer to the poor, but it later took on a more specific meaning, referring to beggars.
According to Michel Mollat's The Poor in the Middle Ages: An Essay in Social History, this was one of a number of general terms used to refer to the poor:
words referring to impecuniosity and destitution in general (egens, egenus, indigens, inops, insufficiens, mendicus, miser)
There are many more specific words that could fit in the categories under the Poor Law system:
- The impotent poor: Infirmus could be used as a general term for poor health leading to an inability to work, while things like caecus (blindness) and claudus (lameness) referred to more specific physical defects.
- The able-bodied poor: This is one of the toughest categories. Penuria refers to simply lacking something, so it could be argued that some uses would refer to the lack of skills needed to work. But it's a stretch.
- The idle poor: Terms for these were not originally common, but by the 1300s, some terms originally used generally for begging took on a negative context and were used here. Mendicus could have had these connotations.
- Vagrants or beggars: See the idle poor with regards to begging. The two categories are fairly closely intertwined, it seems; not much distinction is evident. The poor not deserving of relief were looked upon badly by many. Whether or not they were actually criminals was often insignificant, especially as some started to treat those poor not given relief as criminals regardless.
Mollat found that there are more specific words used to describe how someone came into poverty. Monks who begged voluntarily were referred to using paupertus spontanea (literally, "voluntary poverty"). Women forced into poor living conditions due to childbirth were referred to by mulier ante et post partum. Poverty through slavery or captivity was captivus, and so on.
I originally found Mollat through a mention in The Development Dictionary (page 191, footnote 3).