If you treat “parlous” as a morphological base (even though historically it’s a reduced form of perilous), you can systematically generate prefixal and suffixal derivatives. The distinction is important: most of these are formally valid in English word-formation, but only a subset are historically attested.
Prefix formations (negation, degree, relation):
unparlous — logical negation (not dangerous), but essentially unattested
non-parlous — later, more Latinate negation; not archaic in tone
inparlous / imparlous — theoretically from Latin in-; not used in English here
overparlous — excessively dangerous (constructed; not standard)
underparlous — insufficiently dangerous (constructed; not idiomatic)
outparlous — beyond dangerous (nonstandard)
misparlous — prefix doesn’t semantically fit; effectively unusable
disparlous — not viable; dis- doesn’t pair with this adjective
In practice, English never productively built prefixes onto “parlous.” Writers reverted to peril / perilous instead.
Suffix formations (more structurally natural):
parlously — adverb (in a dangerous manner) — plausible and occasionally seen
parlousness — noun (state of being dangerous) — rare but valid
parlousity — theoretically possible, but not attested; -ity prefers Latinate bases like perilous → perilousness instead
parlousful / parlousless — structurally possible, but linguistically awkward and unattested
parloushood — not idiomatic; -hood attaches to nouns, not adjectives like this
parlousdom — nonstandard formation
Key linguistic constraint
Because parlous is already a phonologically eroded descendant of “perilous,” English morphology “snaps back” to the root when forming families:
peril → perilous → perilously → perilousness (fully productive)
parlous remains lexically isolated, with only light extension (parlously, parlousness)
A writer in the register of John Milton would almost certainly avoid experimental forms like unparlous and instead shift constructions entirely: “secure,” “sure,” or “void of peril.”
So the complete answer is slightly paradoxical:
You can generate many forms mechanically, but historical English recognizes very few of them as legitimate.
