'pro-weed' is a compound formed from the prefix 'pro' (from Latin 'pro', meaning 'for' or 'in favor of') and 'weed' (from Old English 'wēod', meaning 'plant' or 'herb'). The prefix 'pro-' is commonly used in English to indicate support for something, and 'weed' acquired the modern slang sense of 'cannabis' in the 20th century.

'weed' comes from Old English 'wēod' meaning 'plant/herb'; over time 'weed' came to mean an unwanted plant and later became slang for 'cannabis'. The formation 'pro-...' (e.g., 'pro-life', 'pro-choice') has long been used in English to label supporters; 'pro-weed' follows that pattern to mean someone 'for weed.'

Initially the elements meant 'for' (pro-) and 'plant/herb' (weed) in a literal sense, but the compound evolved into a partisan/informal label meaning 'in favor of marijuana or its legalization.'