"online journal," 1998, short for weblog (which is attested from 1993 but in the sense "file containing a detailed record of each request received by a web server"), from (World Wide) Web (n.) + log (n.2). Joe Bloggs (c. 1969) was British slang for "any hypothetical person" (compare U.S. equivalent Joe Blow); earlier blog meant "a servant boy" in one of the college houses (c. 1860, see Partridge, who describes this use as a "perversion of bloke"), and, as a verb, "to defeat" in schoolboy slang. The Blogger online publishing service was launched in 1999.
Old English þrescold, þærscwold, þerxold, etc., "door-sill, point of entering," a word of uncertain origin and probably much altered by folk-etymology.
The first element probably is related to Old English þrescan (see thresh), either in its current sense of "thresh" or with its original sense of "to tread, trample." The second element has been much transformed in all the Germanic languages, suggesting its literal sense was lost even in ancient times. In English it probably has been altered to conform to hold.
Liberman (Oxford University Press blog, Feb. 11, 2015) revives an old theory that the second element is the Proto-Germanic instrumental suffix *-thlo and the original sense of threshold was a threshing area adjacent to the living area of a house. Cognates of the compound include Old Norse þreskjoldr, Swedish tröskel, Old High German driscufli, German dialectal drischaufel. The figurative use was present in Old English.