3. Marked by or showing lethargy, passivity, or blameworthy indifference. See Synonyms at inactive.
4. Inclined; sloping.
n.Grammar
A defective Latin verbal noun of the fourth declension, having very limited syntax and only two cases, an accusative in -tum or -sum and an ablative in -t or -s. The accusative form is sometimes considered to be the fourth principal part of the Latin verb.
[Middle English supin, Latin verbal noun, from Late Latin supnum (verbum), (verb) lying on its back, (verb) going back, neuter of Latin supnus; see upo in Indo-European roots.]