The Greek eklyō means to loosen completely — hence to faint, grow weary, or give out from exhaustion. It appears in Hebrews 12:3 and 12:5, Galatians 6:9, and Matthew 15:32 / Mark 8:3 where Jesus expresses concern for the crowd who have been with Him three days and might 'collapse' on the way home if not fed. The word captures the physical experience of strength running out.
Hebrews 12:3-5 pairs eklyō with the example of Jesus: 'Consider him who endured such opposition from sinners, so that you will not grow weary and lose heart (eklyō).' The antidote to spiritual fainting is fixing our eyes on Christ's endurance of the cross. Hebrews 12:5 then quotes Proverbs 3:11: 'do not make light of the Lord's discipline, and do not lose heart when he rebukes you.' Discipline is evidence of sonship, not rejection. Knowing this prevents the eklyō that comes from misreading suffering as abandonment.