A worthless or stupid person or group.
This is a British term which originated in the late 1980s. It is sometimes spelled without the hyphen, as a single word. Clearly the term is derogatory in its comparison of someone with the invertebrates and insects that are found in garden ponds. The first example that I can find of it in print is from the UK film magazine Empire, September 1989:
"Hollywood's hottest execs: either you're in or you're pondlife."
'Pond life' is at least a little further up the evolutionary chain than the equivalent US insult - 'pond scum'. If the earliest sources that I have are accurate - and they may be superseded of course - the US version marginally pre-dates 'pond life'. The earliest citation I've found for 'pond scum' in this context is from a May 1984 edition of The Washington Post:
"The man I loved is no more than pond scum to me now."