The headword is the present middle/passive participle, masculine nominative singular, of the verb sune/xw, I hold/keep together; see generally LSJ s.v. and cf. cognates at sigma 1523, sigma 1524, sigma 1525, and sigma 1527.
It is presumably extracted from somewhere, but is too common for the source to be identified. Here are just three possibilities. (i) Aesop, Fox and Goat 9.1.2 Hausrath and Hunger: the goat, sunexo/menos with thirst, ventures up to the well in which the fox has become trapped. (ii) Plato, Gorgias 512A (web address 1): Socrates presents Callicles with the example of someone sunexo/menos by bodily afflictions but then contrasts this with a person having an incurable malady of the soul. (iii) Jeremiah 23.9 LXX: simile of a man sunexo/menos by wine.
[1] The first gloss is the present active participle, masculine nominative singular, of the contract verb a)rrwste/w, I am unwell; see generally LSJ s.v. The second gloss is the same form as the headword, from the contract verb krate/w (I am strong, powerful; I conquer), but which in the passive voice acquires the sense of being overpowered; see LSJ s.v. The headword is identically glossed in the Synagoge and Photius' Lexicon; cf. Hesychius sigma 2573-74 s.vv. sunexo/menai (present middle-passive participle, feminine nominative plural) and sunexo/menon (headword form: neuter). [In her critical apparatus Adler reports that ms A omitted the first glossing participle and the following conjunction.]