'Lemmaroo' is a portmanteau of 'Lemmy' and 'switcharoo'. The latter word is the name that was given to a long-standing tradition, originating from Reddit, wherein users try to form a long chain of linked comments across posts in various communities wherein a play on words has been made based on someone's unintentional ambiguity.

If you still use reddit, r/switcharoo explains it perfectly, the key takeaway being:

A switcharoo occurs when a witty [lemmy user] feigns ignorance about which of two subjects in a post (typically an image) is more comment-worthy.

These 'roos are linked in a chain; this [community] keeps track of the most recent link.

If not, here's a summary of the key rules:

Rules of the game:

• Someone makes a comment which has some unintended ambiguity, and another replies with a joke based on the ambiguity.
• You then reply with “Ah, the old switcharoo” (or {thing}aroo), and include a link to the most recent switcharoo comment, which is the latest post in this community.
• Then, post a link to your comment here to grow the chain. Please follow the convention of: "Switcharoo: Thing A vs. Thing B" when titling your post. You may want the link to point to a comment one or two levels above your switcharoo so readers have enough context to get the joke.
• As per the tradition, others can then reply to your Lemmaroo comment with "Hold my {subject of the ambiguity}, I'm going in!", and then "Hello future people!".

Example comment thread:

User 1: "I can't get my kids to eat their vegetables" (introduces the subjects - the kids and the vegetables)
User 2: "You should eat them then" (ambiguity - the kids or the vegetables?)
User 3: "Are you telling OP to eat their kids?!" (follow-up)
User 4/YOU: "Ah, the old [switcharoo](link to latest post here)" (this is the roo comment)
User 5: "Hold my kids, I'm going in!"
User 6: "Hello future people!"

Community rules:

• Don't spam and be nice
• Make sure it's not the same user making the set-up and the punchline, and ensure both subjects involved in the switcharoo have been correctly introduced
• Title your posts as per the convention of: "Switcharoo: Thing A vs. Thing B"
• Always link to the latest post so the chain doesn't branch out
• Mods have ultimate say over what is or isn't a roo
• Only link posts please