PWS

*ṯVkm- - the lowest part of the neck (SED I No. 281; Kogan 2011: 216)

PES

*skm - to carry on shoulders (CDG 496; Kogan 2011: 216)

Ugaritic

ṯkm - shoulder (WUS 334)

Hebrew

šəkäm - shoulder, nape of the neck, back (HALOT 1494)

Demotic Aramaic

tkmh - back (DNWSI 1266)

Arabic

ṯuknat- - pearl necklace; dyed wool collar that hangs as an ornament on the camel's neck (BK 1 231)

presumably related with assimilation of *m > n after the velar. As for the semantic shift, see SED I, No. 15