To inconvenience a stranger on the streets by asking them to stop. This is often used by street salespeople trying to sell their product, people asking for spare change "for the bus", or someone asking for directions.
Generally reactions to stecking vary depending on the stecker's appearance. People are less likely to stop for anyone with a clipboard, a table full of homemade merchandise or anyone sitting at the side of the road in a pile of their own vomit.
"I'm going to steck this woman."
"I got stecked by a tramp asking for money."
"He was stecking people with flyers."