an absolute master: the one who owns you outright and whose authority over you has no limit
In ordinary Greek life this was the plain word for the man who owned slaves. It named a legal fact, not a mood: the slave belonged to him, and what he decided went. Half of the ten times it appears in the New Testament it means exactly that, in instructions telling Christian slaves how to regard the men who owned them.
The other half lift the same word to God without softening it. Simeon, holding the child, calls himself a slave and God his owner (Luke 2:29). The church at prayer uses it in Acts 4:24, the martyrs under the altar use it in Revelation 6:10, and both Peter and Jude use it of the Master who bought his people. Calling God this was not flattery. It was a way of saying that the speaker was owned and that the one who owned him answered to nobody.
Greek had a much broader word for "lord" (κύριος), which the New Testament uses hundreds of times and which can even be a polite "sir". This word is narrower and harder, and it is picked on purpose at moments when the point is belonging rather than rank. Most English Bibles print both as "Lord", so the difference rarely survives the crossing.