The word *landlubber*, first recorded in the late 1690s, is formed from *land* and the earlier *lubber*. This *lubber* dates from the fourteenth century and originally meant 'a clumsy, stupid fellow; lout; oaf'. By the sixteenth century it had developed the specialized sense 'an unseamanlike person; inexperienced seaman', which is the same sense as *landlubber* and was eventually combined with *land* to emphasize the unfamiliarity-with-the-sea aspect.
*Lubber* itself is probably related to or derived from *lob*, a word also meaning 'a clumsy, stupid fellow; lout', which is chiefly an English dialect form but occasionally appears in America (for example: "He is generally figured as nothing but a lob as far as ever doing anything useful...is concerned" -- Damon Runyon). Though *lob* is not found until around 1500, somewhat later than *lubber*, *lob* is clearly related to words in other Germanic languages meaning 'a clumsy person'.
From The Mavens' Word of the Day (October 9, 1997)