蹭 has two common colloquial uses: physical rubbing/scraping and freeloading. 蹭饭 (mooch a meal), 蹭网 (use someone's Wi-Fi for free), and 蹭热度 (ride on something's popularity) are very common expressions.
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蹭 in the freeloading sense is very colloquial and informal. It's common in everyday speech but rarely used in formal writing.
