συμμαρτυρέωsymmartyréō

to bear witness with: Romans 2:15; with dative, Romans 8:16; followed by ὅτι, Romans 9:1.

Greek (New Testament) · G4828

Same root

σύνsýn · G4862with or together (but much closer than G3326 (μετά) or G3844 (παρά)), i.e. by association, companionship, process, resemblance, possession, instrumentality, addition, etc.
μαρτυρέωmartyréō · G3140to be a witness, i.e. testify (literally or figuratively)

Where it appears

Appears 3 times in the Bible.

Romans 2:15 They demonstrate that God’s law is written in their hearts, for their own conscience and thoughts either accuse them or tell them they are doing right.
Romans 8:16 For his Spirit joins with our spirit to affirm that we are God’s children.
Romans 9:1 With Christ as my witness, I speak with utter truthfulness. My conscience and the Holy Spirit confirm it.

Source: G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), public domain.