Jamieson, Fausset and Brown on Jeremiah 13:27

27. neighings--(Jr 5:8), image from the lust of horses; the lust after idols degrades to the level of the brute.

hills--where, as being nearer heaven, sacrifices were thought most acceptable to the gods.

wilt thou not ... ? when--literally, "thou wilt not be made clean after how long a time yet." (So Jr 13:23). Jeremiah denies the moral possibility of one so long hardened in sin becoming soon cleansed. But see Jr 32:17; Lu 18:27.