R. S. Ginzburg, S. S. Khidekel, G. Y. Knyazeva, A. A. Sankin a course in modern english

§ 13. Historical Changeability of Word-Structure

Neither the morphemic nor the derivational structure of the word remains the same but is subject to various changes in the course of time. Changes in the phonetic and semantic structure and in the stress pattern of polymorphic words may bring about a number of changes in the morphemic and derivational structure. Certain morphemes may become fused together or may be lost altogether. As a result of this process, known as the process of simplification, radical changes in the structure of the word may take place: root-morphemes may turn into affixational or semi-affixational morphemes, polymorphic words may become monomorphic, compound words may be transformed into derived or even simple words. There is no doubt, for instance, that the Modern English derived noun friendship goes back to the Old English compound frēōndscipe in which the component scipe was a root-morpheme and a stem of the independently functioning word. The present-day English suffixes -hood, -dom, -like are also known to have developed from root-morphemes. The noun husband is a simple monomorphic word in Modern English, whereas in Old English it was a compound word consisting of two bases built on two stems hus-bond-a.
Sometimes the spelling of some Modern English words as compared with their sound-form reflects the changes these words have undergone. The Modern English word cupboard judging by its sound-form ['kAbэd] is a monomorphic non-motivated simple word. Yet its spelling betrays its earlier history. It consisted of two bases represented by two monomorphic stems [kAр] and [bo:d] and was pronounced ['kAp,bod]; it signified
¹ Though no figures for verbs are available we have every reason to believe that they present a similar relation.
² We may presume that a similar if not a more striking difference is true of verbs, adverbs and all form words.
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'a board to put cups on’; nowadays, however, having been structurally transformed into a simple word, it denotes neither cup nor board as may be seen from the phrases like* boot cupboard, a clothes cupboard. A similar course of development is observed in the words blackguard ['blæg-a:d] traced to ['blæk,ga:d], handkerchief ['hæŋkэt∫if] that once was ['hænd,kэ:t∫if], etc.
In the process of historical development some word-structures underwent reinterpretation without radical changes in their phonemic shape; there are cases when simple root-words came to be understood as derived consisting of two ICs represented by two individual items, e.g. beggar, chauffeur, editor. The reinterpretation of such words led to the formation of simple verbs like to edit, to beg, etc.