In Ireland, it means 'in contact' as in 'was he onto you?' ; it can also mean 'catching someone out' as in 'I was onto him' as in ' I knew what he was up to.' To get onto someone' means to give out to someone.'
Get the Ontomug. Someone kissing and making out with you from behind, but the front persons arms are up and shaped into ears.
Get the Mickey Mouse Runaroundmug. 
