Senseless means having no purpose, or knocked unconscious. A senseless killing, beaten senseless, senseless violence.
The first sense is a judgement about pointlessness and it attaches most often to violence — senseless is what people say when there is no explanation to give. The second is literal and old: without the senses. Note the -less ending meaning without.
1adjectivepurposeHaving no purpose.
“It seems senseless to repaint the fence when we're moving next month.”
2adjectivesenselessKnocked unconscious.
“The falling branch hit him on the head and knocked him senseless.”
senseless is a quality of pointlessness.
Sense plus -less, from Latin sensus, feeling or perception, from sentire, to feel.
From sense + -less. Compare stq sinloos (“senseless, foolish”).
Etymology from Wiktionary, CC BY-SA