This is the combination of two Aramaic words, either מרנא תא (“marana tha”) or מרן אתא (“maran atha”) and means either “Come our Lord” or “Our Lord has come”. It occurs once in the New Testament, in 1 Corinthians 16:22.
If anyone has no love for the Lord, let him be accursed. Our Lord, come! (1 Cor 16:22)
The words in bold are a translation of maranatha.
In this verse, Paul uses a deliberate alliteration (anathema maranatha), which was transliterated in the King James Version thus capturing the sound of the text but obscuring its meaning.
The word also occurs in the first century Christian document known as the Didache:
.. Let grace come, and let this world pass away. Hosanna to the Son of David! If any one is holy, let him come; if any one is not, let him repent. Maranatha. Amen. (Didache 10.6)