'Dirtsider', like the alternative epithets 'mudballer', 'toker', and 'hokie' is a pre-nanoswarm term for (usually unskilled and poorly equipped) immigrants and refugees from Earth.
While terms like Dirtsider and Mudballer can be derived as derogatory expressions of planet-based lifestyle, the etymology of the other appellations are less clear, and the meaning has long since been lost. Many of these terms are thought to have originated from 2nd and 3rd generation orbitaller culture in the 3rd century a.t. With the upgrading of space elevator carrying capacity in the early to middle 3rd century, fall in travel costs, and increase in cislunar infrastructure, and encouraged by misleading "get rich buy high" schemes and utopian visions by space development corporation advertisements, more and more Terrans (note, originally this term referred to people of Earth only, as opposed to Lagrangists, Martians, Lunarians, etc.) sought a better life "beyond the well" (origin unknown, possibly a poetic reference to Earth's gravity well). However, the space development advertising worked too well, and the "boom town" rush of the high frontier meant that many more people arrived than the current habitats could support. Add to this the growing hostility from the original Orbitalers, who resented this new invasion and the strains it placed on the cislunar infrastructure.
The situation was further exacerbated with the closing of Martian borders to all but the most highly skilled immigrants at the end of the Martian War and with the declaration of the new and xenophobic First Mars Republic (unrelated to the post-nanoswarm "Mars Republic"). The terrans were left stranded, and many ended up in the notorious L4 ghettos, hastily built and often substandard habitats where overcrowded conditions, underpaid work, and sometimes even illness and malnutrition, were the rule. Others migrated to those few of the better orbitals that would admit them, and many more were moved in illegally by the people smuggling rings that sprang up like nano-replicators at the time. Others were able to pool their resources enough to buy passage to the Belt, but even here not infrequently the promised share of asteroid turned out to be already owned (see "Doubleclaiming").
Dirtsiders continued to have a destabilizing effect and cause headaches for non-terran polities right up until the nanoswarm period, and in many ways were the interplanetary equivalent of the Terran refugee problem that became chronic from the Information Age onwards. While the small number of prosperous terran governments genuinely cooperated with non-terran polities, the governments of the poorer and more overcrowded Earth polities - whose people constituted the great majority of dirtsiders - were in no hurry to do anything about the problem, as more people leaving meant less to contend with at home.
Some historians and cliologists have suggested that dirtsider dislike was one reason why many space based polities and individuals did not show more sympathy during the Great Expulsion. Indeed, for many, the unfortunate masses expelled from humankind's homeworld by GAIA were just one more wave of mudballers, hence the nanoswarm-era term "the Muddy Billions".