A Grammatical Dictionary of Botanical Latin

dubius,-a,-um (adj.A): dubious, doubtful, uncertain, ambiguous; see uncertain; [cf. varius, anxius, etc. (Glare) “moving in two directions alternately, vibrating to and fro, fluctuating; vacillating in mind, uncertain; wavering in opinion, doubting, doubtful, dubious, uncertain; irresolute, undecided” (Lewis & Short); see uncertain;

dub. abbrev.: dubius,-a,-um (adj.A), dubious, uncertain, ambiguous;

- Synon. dub., synonymia dubia, the synonymy uncertain.

- dub.: dubia Synonyma, synonyms uncertain (Dillenius).

The following after Glare:

(of actions) hesitant, undecided, wavering.

(of a person) uncertain what to think, doubtful; untrustworthy, unreliable; of questionable character.

Poised between movement in one direction or another, tottering, uncertain (as the weather: changeable, fitful);

(of situations, etc) having a doubtful issue; unpromising.

Not certainly in a particular state, faint, uncertain (obscure); not certainly perceptible to the senses.

Having a dual nature, potentially good or bad, as oracles ambiguous, equivocal, concerning which doubt is felt, uncertain, doubtful, strange;

- extra dubium: out of doubt, beyond doubt, without doubt, doubtless.

- in dubium: in question, in doubt, lacking certainty.

- in dubio: in doubt.

- sine dubio: without doubt, certainly, with certainty.

- species dubiae, doubtful species.

- species sedis dubiae (incertae sedis), a species of uncertain position (Stearn).

- [moss] frigore ingruente, omniumque vegetabilium fibris rigentibus muscum nullum florere extra dubium est (Brid.), when the cold is coming on, and when the fibers of all plants are hardened [i.e. rigid with the cold] then it is beyond doubt that no moss is flowering.

- [Synalissa meladermia; lichen] Simul initia Scytonematum inspersa adsunt, quare haec formatio haud parum magma dubium fìngere videtur (Nyl.), simultaneously, the [cell] initials of Scytonema are present, interspersed, for which reason this formation [i.e. development] seems to create a not insignificant curious [i.e. strange] ointment [i.e. a smooth, oily lotion or soft grease; a gelatinous substance].

- semina túrgida statu casso tantum visa, qua re genus dubium remanet (F. Mueller), seeds swollen, with only the empty state seen, for which reason the genus remains dubious [i.e. doubtful, uncertain].

- [Phascum dubium; moss] forsan Ph. cuspidati varietas! (C. Muell.), possibly a variety of Phascum cuspidatum!

[moss] Fissidens dubius P.Beauv., from the Latin adj. dubius, ambiguous, uncertain, doubtful; the author of this species collected the type in the United States but it was sterile (“not in flower”). He suspected it was a variety of Fissidens subbasilaris Hedw., but Hedwig omitted to say whether the leaf margins were serrate. The species seems to be intermediate between F. subbasilaris and F. cristatus (a synonym of F. dubius) hence the ambiguity.