Hitler's foreign policy stemmed from two ideologies evident in German political discourse since the nineteenth century, "Weltpolitik" and "Lebensraum". Weltpolitik held that Germany's foreign policy should be dictated primarily by economic gain and the empire should adopt an informal imperialistic approach wherever possible, whereas Lebensraum emphasised the importance of formal settlement colonies, in part to stem the tide of large-scale emigration of Germans to their economic rivals. These two ideas were largely contradictory, yet both held increasing sway in German politics throughout the nineteenth century and were a significant factor (albeit just one in a complicated process) behind Germany's establishment of overseas colonies in the 1880s. Nevertheless, Eastern Europe was seen by many as a vital frontier for German expansion. Proponents of Lebensraum frequently pointed to this comparatively sparsely populated land area and drew comparisons between it and the American West. Hitler's idea to depopulate Eastern Europe for German settlement therefore drew on this century-long concept of Lebensraum that was already popular in Germany.
Edit: Added some useful literature on the topic