
Not much subtlety here. A strong arm, or Arm-Strong if you will. The ‘Invictus Maneo’ literally translates as ‘I remain unvanquished’, but there’s some Latin punning going on here. First, maneo ‘I remain’ is invoking another Latin word ‘manus’ meaning ‘hand’. Second, and much less subtle, the fuller motto of the Armstrongs is ‘Vi et arms Invictus maneo’ – through STRENGTH and ARMS I remain unconquered.’
This Armstrong Crest is first recorded in Alexander Nisbet's System of Heraldry of 1722. In this he describes the arms of the senior family, the Armstrongs of Mangerton with this crest, and the cadet branch of Whittock, who seem to have supplied the motto. So this is something of a composite, but has been in use for a long time to represent this kindred.
There are variants of the Armstrong Crest. Our old line of Carrick badges, designed in the middle of the twentieth century, has a plain arm with the same motto for Armstrong. This seems to resemble the Armstrongs of Whittock in purer form, as recorded in Nisbet:

The Carrick version of the Armstrong Crest with unarmoured arm.
MKP 24 June 2020