Land sakes! < Living Saints!
1. Living Saints > living sangs* by phonetic attraction of nts in Saints to ng-S in livi[ng S]aints!
2. Living sangs* > liven’ sanks* by phonetic compensation of ngs to nks in sanks* for assimilation of ng to en in liven’ before s in sangs.* The progress from initial living to contracted liven’ with vocalic reduction of i to sluggish e in en reflects typical vulgarity in spoken English.
3. Liven’ sanks* > liv’n snakes by sequential contraction of final en to n in liv’n and grammatical metathesis of an in sanks* to na in snakes—cf. "Snakes alive!" < "Saints alive!" But the cluster vnsn formed by li[v’n-sn]akes is in itself prohibitive to articulate.
4. Liv’n snakes > land sakes by compensatory lengthening of i to diphthong [ai]*—after nature of English—to make up for loss of v in liv’n. Said vowel may then become short a upon insertion of euphonic [d] before s in sakes, but not until after n in snakes has already been lost by rule of dissimilation with preceding n in land. Finally, that miscreant expression of surprise—"Landsakes!" For similar development: Cf. "for Godsakes!" < "God’s Saints!", which has clearly been attached to the preposition for rhetorical effect!
*hypothetical explanation
